This invention relates to industrial and comfort air conditioning systems and, more particularly, to air conditioning systems having controls for automatically operating the dampers in the outside-air duct and the return-air duct.
Typically, air conditioning systems used in manufacturing facilities have the size and capacity to maintain the inside air at a dry bulb temperature of about 75.degree. F. and a relative humidity of about 60%. As shown in standard ASHRAE psychrometric charts, such inside air conditions may be attained by using supply air having a temperature of about 61.5.degree. F. and a relative humidity of typically 95%. Such supply air conditions can be achieved by mixing outside air and return air when the outside conditions or heat content (enthalpy) is below the required supply air. On the other hand, when the outside air conditions or heat content rises above the required supply air conditions or heat content then mechanical refrigeration, or mechanical energy, is required for the cooling medium.
In air conditioning systems, so equipped, dampers control the flow of inside air and outside air to the cooling coil or cooling medium. As between the use of outside air and the return air, it is clearly more energy efficient, during mechanical refrigeration periods, to pass over the cooling coil or cooling medium the air (outside or return) which has the lowest heat content. Control systems have thus been developed to make maximum use of the outside air when its heat content is low and to make maximum use of the return air when its heat content is lower than that of the outside air. One such system is disclosed in U.S. Pat. No. 2,206,445; another is referred to in the background section of U.S. Pat. No. 4,347,712.